Aromatic ketones which may be used in a variety of chemical reactions have been prepared by various alternate reactions. For example, one method of preparing an aromatic ketone is by a Friedel Crafts acylation reaction in which benzene and an acyl chloride such as acetyl chloride or propionyl chloride are reacted in the presence of a Friedel Crafts catalyst such as aluminum chloride, boron trifluoride, etc. Another method of preparing an aromatic ketone has been an air oxidation reaction in an oxygen-enriched environment utilizing a transition metal catalyst such as cupric bromide at relatively high temperatures and pressures. Likewise, aromatic ketones may be synthesized by the chemical oxidation of the substrate using stoichiometric quantities of electrochemically generated oxidants such as salts of cobalt, manganese or chromium in their highest valence state in a strongly acidic medium at elevated temperatures. The reduced oxidant is then recycled, purified and electrolytically reoxidized back to its active state.
The inherent drawback in the last named reaction involves the reoxidation and recycling of the oxidant by electrochemical methods. Heretofore, all of the methods which have been employed in this area have oxidized the transition metal to its higher valence state prior to combination of the same with the organic substrate in a conventional chemical reactor. In essence, this comprises a two-step reaction which requires both an electrochemical reactor and a chemical reactor. In addition, the aforementioned processes have utilized relatively concentrated acids such as from 40% to 70% concentration of sulfuric acid or perchloric acid, thus making the selectivities of these processes for activated alkyl aromatic systems less than desirable. The undesirability of these processes results from the tendency of the alkyl aromatic systems towards sulfonation or by-product formation.
Prior work in the oxidation of aromatic compounds has been shown in U.S. Pat. No. 4,046,652. However, this patent describes the oxidation of an aromatic nucleus in an electrochemical reaction to form p-benzoquinone diketals. The electrolyte which is used in this electrochemical reaction comprises methyl alcohol containing a conducting salt, preferably an ammonium or alkali metal salt of an acid such as hydrofluoric acid, perchloric acid, nitric acid, etc. Likewise, U.S. Pat. No. 4,148,696 also relates to electrochemical oxidation reactions involving aromatic compounds. However, this patent involves an anodic acyloxylation involving the use of a salt of a fatty acid. Another patent, namely U.S. Pat. No. 4,101,392 discloses a process for the electrolytic oxidation of aromatic compounds. However, this patent is concerned with a process for the methylmethyl coupling of hydroxy aromatic compounds, which process is in contradistinction to the process of the present invention, hereinafter set forth in greater detail, which is concerned with the oxidation of the methyl substituent of a methyl-substituted aromatic compound. An article which appeared in the Bulletin of the Chemical Society of Japan, volume 37, number 11, has disclosed an electrochemical process for the methoxylation of aromatic compounds. This anodic oxidation was effected by treating an aromatic compound such as tetralin, indane, or diphenylmethane, to afford a methoxy-substituted aromatic compound. However, this process is dissimilar from the process of the present invention in which an alkyl-substituted aromatic compound is converted to a ketone at the benzylic carbon atom.
U.S. Pat. No. 4,284,825 teaches the preparation of a 4-substituted benzaldehyde-dialkylacetal by the electrochemical oxidation of a 4-substituted methylbenzene using an electrolyte consisting of an alcohol which contains a conductive salt. However, in contradistinction to this process, as will hereinafter be set forth in greater detail, it has now been discovered that the oxidation of alkyl aromatic compounds in which the alkyl substituent contains at least 2 carbon atoms in an electrochemical reaction may be effected in an electrolyte which includes the presence of a nucleophile of the type later set forth to form an intermediate compound which may then be converted to the desired ketone.